So, in my story, there is a city that gets cut off from the rest of the world. Golden translucent walls rise up on all four sides, going all the way up into the atmosphere and all the way down to the earth's core. And while most matter can pass through these walls, it swiftly becomes apparent that living humans cannot.
The city is trapped in what will come to be known as a "square", a roughly 2x2 mile column of the earth that is subject to a supernatural gimmick. The gimmick here is that, in addition to electricity not working properly, the golden walls of the square stop letting living humans pass through them (neither in nor out) whenever there are more than 100 living humans inside of it. As a result, once the people inside figure this out, life inside the square steadily devolves into a battle royale where everyone tries to ensure that they and their loved ones are among the last 100 standing who are allowed to leave, all before the people inside all starve to death.
The key factor here being "once the people inside figure this out". While most of the time the effects of squares will be completely unknown until discovered through (frequently lethal) trial and error, in this case the purpose of the square needs to be more obvious in order for it to serve its narrative purpose. I need people to be able to figure out the escape conditions of the square, because the entire narrative hook of the setting depends on the people inside realizing that they have to kill each other if they ever want to leave. If nobody ever figures that out, the events inside the square will play out very, very differently.
However, I don't want to accomplish this by simply having writing emblazoned on the walls of the square saying "the walls will remain closed until only 100 humans remain alive". To keep the origins and purpose of the squares vague and open to interpretation (whether it be an act of god, aliens, or seemingly-intentional chaotic magical forces), I want to avoid having the squares have any blatant "built in" explanation of how they work as much as possible. Nothing that makes it seem like somebody designed it specifically so that the people inside would find out. But that makes giving the people inside a way to figure out how it works... rather difficult.
What element can I add to the square as I have described it that would allow people to figure out the escape conditions of the square within a few days to a week, all without making it seem like that element was put there specifically to ensure that the people inside would figure it out?